Information from sources, such as, for example, the Internet, or other sources, may be very dynamic over time. For example, a news source may add and update articles. A search using a set of query terms may return different results over time because new Web pages may be added, changed, and deleted, and changes to algorithms for computing relevancy of search query results may be modified. Articles on sites, which rely on solicited contributions or corrections, such as, for example, Wikipedia, or similar sites, may change over time.
Users frequently request and view the same content. Users may view the content weekly, daily, or multiple times per day. Generally, users are most interested in changes to the content. For example, when viewing a news site, users are interested in most recent news articles. When visiting an auction site, users wish to determine whether a high bids changed on items of interest. If the content has not been viewed for a long period of time, changes to the content since a last viewing may be pervasive making it difficult for users to determine exactly what changed and what remained unchanged.